Specific aims of the proposal which should provide new information and insight into some fundamental aspects o the way in which transferrin interacts with its receptor include: 1) Use of differentially radiolabeled monoferric fragments or "half" molecules of Tf as a probe of the Tf-receptor interaction in a chick reticulocyte system with particular application to the mechanism of iron removal. The half molecules are physiologically active only when both are present. 2) Isolation and physical characterization of the chick reticulocyte transferrin receptor and definition of the stoichiometry of transferrin and the receptor. 3) Development of a noncompetitive immunoradioassay (IRA) for the Tf receptor allowing comparison of the amount of receptor found by the IRA at different stages of development with the amount of receptor found by the usual binding type assay as a method of testing whether there is a pool of receptor unavailable for binding. 4) Investigations of the topography of Tf vis-a-vis its receptor by use of a new, well controlled, cross-linking reagent which allows radiolabeling of macromolecules in close proximity to the derivatized probe and by surface iodination of bound Tf in combination with peptide mapping to allow definition of the areas of receptor-ligand interaction. Iron is critical to the metabolism of all cells. The essential role played by transferrin and the transferrin receptor in the delivery of iron to cells has recently become more apparent. Transferrin is one of a few factors needed for serum-free growth of animal cells in culture. Furthermore the appearance of transferrin receptors is a prominent feature of cell transformation. In view of the universality of the Tf-receptor complex in iron delivery, it seems important to define the interaction at the molecular levels as completely as possible. Such information might ultimately be useful in understanding diseases involving dyscrasias of iron metabolism, e.g., thalassemia, sickle cell anemia and hemochromatosis.